1. Field of the Invention
The present invention relates to a digital television system (DTV) and more particularly to determining co-channel interference in the DTV employing a vestigial sideband (VSB) transmission system.
2. Discussion of Related Art
According the digital television broadcasting standardized by the Grand Alliance (GA), both the digital and NTSC television systems use radio frequency (RF) signals. Also, each channel band of the digital and NTSC television system is limited to 6 MHz. However, a significant difference between the digital and NTSC television system is in the transmission spectrum of the broadcasting.
In the transmission spectrum of the NTSC system, a large video carrier frequency is always 1.25 MHz above the low end of a channel, a chrominance subcarrier frequency is 3.5 MHz above a video carrier signal, and a sound carrier signal has an unmodulated center frequency of 4.5 MHz higher than a video carrier frequency for the same channel. In contrast, the transmission spectrum of the DTV system only has a very small pilot carrier signal with an unmodulated center frequency of 0.31 MHz to assist the carrier recovery at the receiver.
Moreover, DTV signals are coded and transmitted in a different manner from the coding and transmission methods of the analog signals for NTSC system. Thus, a viewer generally cannot watch a NTSC broadcast with the DTV system. Nevertheless, a same tuner may be used since the DTV and NTSC system both use the same RF channel and the same intermediate frequency (IF) band. The DTV signal and the conventional analog NTSC television signal use the same RF channel and the NTSC system uses a portion of the channel designated as a taboo channel to prevent channel interference. Thus, the DTV and NTSC system may share an antenna and a tuner.
However, co-channel interference may occur in the DTV broadcast, when an NTSC broadcasting is utilizing the co-channel during the DTV broadcasting. The carrier powers, namely the video carriers, the chrominance subcarrier, and the sound carrier, of the NTSC system are significantly larger than the pilot signal of the DTV system, resulting in severe interference in the DTV broadcast. Generally, a comb filter is used to remove co-channel NTSC interference.
The comb filter consists of delays and subtracters. Each delay consists of 12 resistors connected in series and delays the received signal for 12 symbols to obtain the transmission characteristics suitable for filtering. The comb filter also subtracts the delayed data for 12 symbols from the current data. The carrier powers of the NTSC system are removed by the comb filter by converging to "0" (zero). Specifically, the video carrier, chrominance carrier, and sound carrier of the NTSC system are around the null point when passing the comb filter, thereby avoiding interference with the DTV system.
Using a comb filter also increases the power of the random noise resulting in a deterioration of the picture quality. However, because the deterioration of the picture quality is insignificant compared to the effect of removing the NTSC channel interference, using a comb filter is still preferable. Furthermore, since the co-channel NTSC interference does not always exist, the comb filter is used when a co-channel NTSC interference occurs in the DTV system.
FIG. 1 is a block diagram of the DTV system. When RF signals modulated according to the VSB system are received through an antenna 100, a tuner 110 selects a desired channel frequency by tuning and converts the frequency into an IF signal. A frequency-phase locking loop (FPLL) 120 demodulates the IF signal from the tuner 110 into baseband signals I and Q to lock the output phase and frequency. The FPLL 120 is a circuit unifying both a frequency tracking loop and a phase-locking loop, but the frequency is locked prior to the phase.
An analog/digital (A/D) converter 130 converts the I signal into digital data of a given bit number (for example, 10 bits). The Q signal is used for carrier restoration within the FPLL 120. Using the converted digital data of a given bit number, a synchronization recovery unit 140 recovers a synchronization signal inserted at the time of transmission to be used for timing recovery and equalization.
The standardized VSB transmission signal includes a specific synchronization (sync) signal pattern within the transmitted data which allows a receiver to easily recover the sync signals. Accordingly, the data incorporates consecutive data segments, wherein each segment consists of 832 symbol units and within the 832 symbol units, four symbols define the segment sync signal pattern while the other 828 symbols are data information. Also, a field consists of 313 data segments and at every 313th data segment position, a field sync signal is inserted. The segment and field sync signals are similar to a horizontal and vertical sync signals of the NTSC broadcasting signals, respectively. Because the sync signals allow an easy data recovery, erroneous detection and recovery of the sync signals adversely affects the overall system.
An equalization & error coding correction (ECC) unit 150 uses the data segment and data field signals as training signals. An equalization is performed to correct linear distortion of amplitude causing interference between symbols, ghost images, etc. Afterwards, an ECC is performed to correct errors that may have occurred during transmission through the channel. A video decoder 160 decodes the equalized and error corrected signal according to the moving picture expert group (MPEG) algorithm, allowing the TV audience to watch a broadcast through display 170.
Before the sync recovery unit 140 outputs the recovered sync signals to the equalization & ECC unit 150, a determination unit 190 determines whether co-channel NTSC interference exists. If co-channel NTSC interference does not exist, the data bypasses a comb filter 180 and is output to the equalization & ECC unit 150. If co-channel NTSC interference is determined to exist, the data passes through the comb filter 180 and is output to the equalization & ECC unit 190. The determination unit 190 controls whether the data passes or bypasses the comb filter 180 by sending a selection signal to a selection unit 200 which outputs a comb filtered data if the selection signal is received and outputs the data directly otherwise.
The DTV and NTSC broadcasting systems transmit data through space and may be significantly attenuated by various noise in space. The noise which is picked up by a channel during transmission is a Gaussian noise. The power of the Gaussian noise is generally constant, but may sometimes surge instantaneously to a large value. In such cases, the Gaussian noise may be determined to be co-channel NTSC interference causing the data to pass through the comb filter 180 before being output to the equalization and error code correction unit 150.
Particularly, in a zone where the DTV and NTSC broadcasting system overlap, a common-channel interference such as the Gaussian noise or a co-channel NTSC interference may occur. When a noise which passes through and which does not pass through the comb filter 180 have a similar size, the corresponding data which passes or does not pass through the comb filter 180 may be incorrectly selected and output, this adversely affects the operation of equalization & ECC unit 150. Therefore, circuits removing noisy signals contained in the incoming digital signals are affected causing a malfunction of the DTV receiver.